1. Field of the Invention
The present invention relates to a flicker improvement effect evaluating system and, more specifically, to a system for evaluating beforehand the effect of improving voltage flicker, for introducing a static reactive power compensator to a power system.
2. Description of the Background Art
A phenomenon called voltage flicker has been known as a form of voltage fluctuation in a power system. Voltage flicker occurs, in a large-scale electric installation such as an arc furnace load, because of abrupt and temporally irregular fluctuation of reactive power component caused by start and stop of operation or load fluctuation of the installation and of negative phase component caused by arm short-circuit. When voltage flicker occurs, general consumers fed from the same substation experiences unsteady shining of illumination load or wobble of TV receiver images, which are uncomfortable.
As a measure for mitigating such voltage flicker and improving voltage stability, an arrangement has been widely used in which a static reactive power compensator compensates for the flicker component (reactive power component, negative phase component), following voltage fluctuation of the power system (for example, see Japanese Patent Laying-Open Nos. 2005-080368, 2003-324847, 2006-352943). By this arrangement, the static reactive power compensator compensates for the flicker component generated by the arc furnace load as the source of voltage flicker, and hence, the voltage flicker can be mitigated.
On the other hand, when the static reactive power compensator is installed in the power system, compensation of the flicker component with as small compensation capacity as possible of the static reactive power compensator is desired, from the economical point of view. Therefore, when planning installation of a static reactive power compensator, it is desirable to evaluate in advance the effect of improving voltage flicker, to maximize the effect of applying the static reactive power compensator with limited compensation capacity.
The voltage flicker, however, varies dependent on where the static reactive power compensator is installed. Therefore, it is difficult to find the effect of improving voltage flicker in a single uniform way by a general calculation. Specifically, a voltage Vf of the power system as the object to which the static reactive power compensator is installed, is represented by Equation (1) below, where If denotes a load current flowing from a bus to an arc furnace load, Zs denotes system impedance of the system upper than the bus, and Vs denotes voltage behind upper power system:Vf=Vs−Zs·If  (1)
As is apparent from Equation (1), the voltage flicker generated in system voltage Vf differs considerably, dependent on difference in system impedance and difference in load current If that depends on type of arc furnace and presence/absence of any other voltage flicker source.
In view of the foregoing, recently, a flicker suppression effect simulator for simulating the effect of suppressing site-specific flicker, such as disclosed in Japanese Patent Laying-Open No. 2006-352943, has been proposed.
Specifically, the flicker suppression effect simulator simulates, based on the voltage of the electric power system and a current as an object of compensation flowing to the flicker source such as the arc furnace installation from the power system through a bus, the not-yet-improved system voltage at present and the improved system voltage as improved by the installation of a static reactive power compensator to the bus to which the current as the object of compensation flows, and from flicker values of the simulated not-yet-improved system voltage and improved system voltage, operates an improvement rate attained by the flicker compensator.
According to the flicker suppression effect simulator described in Japanese Patent Laying-Open No. 2006-352943, in the power system to which the static reactive power compensator is to be installed, the flicker value and the improvement rate when the static reactive power compensator is connected to a prescribed bus can be simulated easily by simply taking in the actual system voltage and the actual current flowing through the object of compensation.
Because of the following reasons, however, the flicker suppression effect simulator described in Japanese Patent Laying-Open No. 2006-352943 cannot very accurately simulate the effect of flicker suppression.
Specifically, the system voltage Vf is determined by the system impedance Zs and load current If, as can be seen from Equation (1). The system impedance Zs is a variable value that changes momentarily in accordance with the power system setup reflecting season or time of day. Though the system impedance Zs can be approximated on the desk by checking state of operation of a generator or generators of the power company, it is not so easy to calculate accurate impedance value on the desk, considering state of power generating operations of neighboring consumers.
Though Japanese Patent Laying-Open No. 2006-352943 discloses an arrangement for simulating the not-yet-improved and improved system voltages based on the system voltage and the current flowing to the flicker source, it is silent about such fluctuation of the system impedance Zs in the process of operating the system voltage. Therefore, the arrangement cannot accurately derive the voltage flicker generated in the system voltage Vf.